Yes, he may file in both countries if a dual citizen or landed immigrant. Also, in Canada the line for "Income" that child support is generally based on is "Line 150".

I assume you are not filling in Canada, but when we calculate child support it's common practice to ask for the last 3 years's income tax returns (Line 150), and the last 3 months income. Assuming both parties work throughout the year (not seasonal workers), child support is based on the last 3 months income and last year's tax return is a mere detail.

I don't know exactly where this guy claims citizenship. That's a good question to ask in his interrogatories. He is originally from Bangladesh. He lived in the U.S. for 20 years and he now lives in Toronto. He owns three Pizza Hut franchises and he is not claiming the income on his U.S. returns.